OCN{ PHYSTCS B (ADVANCTNG PHYSTCS) OXFORD CAMBRIDGE AND RSA EXAMINATIONS Advanced GCE. Field and Particle Pictures. t hour 15 minutes TIME

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1 .A OCN{ OXFORD CAMBRIDGE AND RSA EXAMINATIONS Advanced GCE PHYSTCS B (ADVANCTNG PHYSTCS) Field and Particle Pictures Thursday 16 JUNE 2005 Morning t hour 15 minutes Candidates answer on the question paper. Additional materials: Data, mulae and Relationships Booklet Electronicalculator Candidate Name Centre Number Candidate Number TIME t hour 15 minutes INSTRUCTIONS TO CANDIDATES o Write your name in the space above. o Write your Centre number and Candidate number in the boxes above.. Answer all the questions. o Write your answers in the spaces provided on the question paper.. Read each question carefully and make sure you know what you have to do before starting your answer. o Show clearly the working in all calculations, and give answers to only a justifiable number of significant figures. INFORMAilON FOR CANUDATES. You are advised to spend about 20 minutes on Section A and 55 minutes on Section B. o The number of marks is given in brackets [ ] at the end of each question or part question. r Four marks are available for the quality of written communication in Section B.. The values of standard physical constants are given in the Data, mulae and Relationship Booklet. Any additional data required are FOR EXAMINER'S USE given in the appropriate question. Section Max. Mark A 20 B 50 TOTAL 70 This question paper consists of 14 printed pages and2 blank pages. sp (MML l n O Obn zoos lvhoomlol Registered Charity Number: [TUrn Over

2 2 Answer all the questions. Fol Examinefs Section A Here is a list of units. Choose the correct unit for (a) electric field strength... N C-1 J C-l Wb s-l Wb m-2 (b) magnetic field strength... l2l 2 Fig.2.1 shows the path followed by an alpha particle as it is deflected by a gold nucleus. G-- gold nucleus Fig.2.1 (a) Mark with an arrow the direction of the force on the alpha particle when it is at point X. t1l (b) At point X, the alpha particle is 1.2 x 10-13m from the centre of the gold nucleus. A gold nucleus and an alpha particle contain 79 and 2 protons respectively. Calculate the magnitude of the force on the alpha particle when it is at point X. =1.6x10-1sC 1 k= - =9.0x10eNm2C-2 4neo

3 3 The graph of Fig.3.1 shows how the fluxin the time. 3 secondary coil of a transformer varies with Examinefs +80 flux/pwb _ Fig.3.1 (a) On Fig.3.1, sketch the variation with time of the emf across the secondary coil. There is no need for any calculations. t1l (b) The secondary coil has 75 turns of wire. By measuring a suitable gradient on the graph of Fig.3.1, show that the maximum emf across the coil is about 10 V. t21 Here are some statements about particles in a nucleus. A B C A stable nucleus always contains more neutrons than protons. Emission of a gamma photon always results in a loss of mass for a nucleus. A nucleus can always reduce the number of neutrons by emitting a gamma photon. Which one statement is correct? [Turn over

4 5 Fig.5.1 represents a gold nucleus. Fig.5.1 On Fig.5.1, sketch eight lines to represent the shape and direction of the electric field surrounding the nucleus. I2l 6 The electric potential V"r". ot a point distance r from a positive charge O is given by the expression Velec= KQ r Explain the meaning of the term electric potential. t2l 7 The force F on a wire of length I carrying a current 1 at right angles to a magnetic field of flux density B is given by the equation F = IlB. Show that the units of flux density can be N s C-1 m-1. t2l

5 In a scattering experiment, a beam of particles is scattered by nuclein a thin target. 5 Examiner's All of the particles in the beam have the same mass, charge and energy. Of the three quantities mass, charge and energy, which one, by itself, if increased would result in more particles being scattered out of the beam by more than 90"? answer... t1i An alpha particle has a head-on collision with a gold nucleus. The collision reverses the direction of the alpha particle's velocity. alpha particle II o gold nucleus I.D \_/ On the axes of Fig.9.1, sketch how the kinetic energy of the alpha particle varies with time t, from well before the collision (f = 0) to well after it is over. kinetic energy time of closest approach Fig.9.1 [Section A Total: t3l 201 [Turn over

6 6 Section B Exaninefs 10 This question is about a type of electric motor. Computers are often cooled by fans turned by brushless motors. Details of a simple brushless motor are shown in Fig fixed iron core, not rotating Fig.10.1 To make the cylinderotate around the fixed core, pulses of current are alternately applied to the H and V coils wound around the iron core. (a) lnitially, a pulse of current is provided in the H coil. This sets the magnetised cylinder rotating clockwise, starting from the position shown in Fig (i) Mark on Fig the N and S poles on the fixed iron core required to make the magnetised cylinderotate clockwise. I1l (ii) Explain how the presence of these poles makes the cylinderotate clockwise. t1l

7 7 (b) 1ic keep the cylinder rotating, a series of pulses of current are provided in both H and V coils. Examinefs The graph of Fig shows how the current in the H coil changes with time. current/ma Fi On Fig.10.2, sketch the pulses of current in the V coil which will keep the magnetised cylinderotating. tzl (c) data from Fig.10.2 to explain why the maximum speed of the cylinder will be 25 rotations per second. t2l (d) (i) Suggest and explain one modification to the motor which will make the cylinder rotate faster. (ii) Suggest and explain one modification to the motor which will result in a larger turning force on the cylinder. t2l tzl [Total: 1ti] [Turn over

8 11 This question is aboutriggering the rapid release of gamma photons from a nucleus. 8 Examinet's Fig shows some of the energy levels for a nucleus of hafnium-178. energy/mev 2'5o 2.48 ground state Fig (a) Hafnium-178 is created by bombarding a target of tantalum-181 with highenergy protons from an accelerator. This process is represented by the equation 1!lra + lp + ll8rnt +x Calculate the mass and charge numbers of particle X, and hence identify it. (b) Some of the hafnium-178 nuclei created are in the excited state, labelled M in Fig Nuclein state M decay very slowly, with a decay constant ot7.1 x 1g-10s-l. ln a recent experiment, a sample of 5.0 ng of hafnium-178 was created in state M. (i) Calculate the mass of a single nucleus of hafnium u=1.7x10-27kg 121

9 (ii) Show that the sample contained about 2 x 1016 nucleiof hafnium-178. I Exaninefs (iii) Calculate the activity of the sample. Include the unit of your answer. t1l activity = unit... t3l (c) The sample of hafnium-178 in state M was exposed to a one second pulse of X-ray photons of wavelength 6.2 x 10-11m. (i) Show that the energy of each X-ray photon is MeV. c = 3.0 x 108m s-1 h = 6.6 x 1O-34J s =1.6x10-1eC (ii) The X-ray photons are absorbed by the sample. The energy level K is MeV above the level M. Explain how this fact could account for the absorption of the X-ray photons. t3l (iii) Absorption of the X-ray photons by the sample results in a large increase in activity. Nuclein the energy level K have a large decay constant. Explain how this could account for the increase in activity. l2l tizl [Total: 14] [Turn over

10 10 12 This question is about accelerating particles to high energies. Particles are accelerated in a ring-shaped evacuated tube, as shown in Fig magnet \ ttt to I tube,evacuated a>beamor Protons \7 Fig.12.1 Fig.12.2 (a) A particle accelerator of this type at CERN accelerates protons to a total energy of 27OGeV. the relationship Erest = mcz lo show that the energy 27OGeY is about 300 times the rest energy of the protons. ffin= 1.7 x kg e'= t'o x 1O-1eC c= 3.0 x 108ms-1 (b) Fig.12.2 shows the magnets above and below the evacuated tube. These force protons moving along the tube to follow a circular path. (i) Low energy protons of mass m and charge g move at speed v through the magnetic field of flux density 8. By using the expressions for the centripetal force and magnetic force on the protons, show that the radius r of the circular path is given by the expression mv r- - Bq t3l

11 11 (ii) Protons with a large total energy E move at almost the speed of light c. In these conditions, the radius rof the circular path is given by the expression Examinels cbq Calculate the magnetic flux density required to keep 27QGeV protons in a circular path of radius 1.8 x 103 m. c= 3.0 x 108ms-1 e=1.6x10-1ec flux density =... T [2] (c) In an experiment, protons and antiprotons travel in opposite directions through the evacuated tube. (i) Suggest why protons and antiprotons travel in opposite directions in the evacuated tube. Sometimes, when a proton and an antiproton collide, a particle called the Z is created. The Z particle is unstable and decays quickly into a positron and an electron. (ii) Complete the equation for the decay of the Z parlicle, showing nucleon and charge numbers. 3z+ (iii) The experiment shows that the Zparticle has a rest energy of 93GeV. Suggest how this is determined by the experiment. lll l2l tzl [Total: 12] 286,4/01 Junos [Turn over

12 13 This question is about calculating the risk to workers exposed to radioactive materials. 12 Examiner's Disposable surgical instruments are sterilised by gamma photons from a sample of cobalt-60. The instruments their airtight plastic bags are packed into boxes and placed on the conveyor belt, as shown in Fig cobalt-60 source 10m view from above.f conveyor belt O- operator stands here Fig.13.1 (a) Operators are required to stand on a spot that is 10 m from the cobalt-60 source. Boxes that they load onto the conveyor belt pass much closer to the cobalt-60, and are exposed to a high intensity of gamma photons. The intensity 1of gamma photons at a distance dfrom a source which emits photons at a rate A is given by the expression J= 4nd2 (i) The source emits gamma photons at a rate of 2.4 x 10168q. Show that the intensity of gamma photons for the operators would be about 2 x 1013 Bq m-2 in the absence of shielding. l1l (ii) Explain why the intensity of gamma photons decreases with increasing distance from the source. (iii) A nucleus of cobalt-60 releases a beta particle when it decays, quickly followed by a pair of gamma photons. Explain why the beta particles contribute very little to the absorbe dose of the operator. tzl t1l

13 13 (b) ln order to reduce the intensity of gamma photons for the operators to a safe level, a 1.2 m thick wall of concrete is placed between them and the source, as shown in Fig Examinefs cobalt-60 source concrete conveyor belt operator stands here Fig.13.2 (i) The intensity of the photons is halved for each 4.0 x 10-2m thickness of concrete that they pass through. Show that the 1.2m thickness of concrete reduces the intensity of gamma photons for the operator to about 2 x 104 Bq rn-2. l2l (ii) Each operator presents an average area of 0.80 m2 for absorption of the gamma photons. lf all of the photons are absorbed by an operator, show that the operator absorbs about 1 x 10-4J of energy in each eight hour working day. energy of photons = 1.8 x 10-13J (iii) The average mass of an operator is 75 kg, and the gamma photons have a quality factor of 1. Show that the absorbed dose of the operator is below the recommended safe limit of 4.0 x 10{Sv per day. tzl END OF OUESTION PAPER l2l [Total: 10] Quality of Written Communication [4] [Section B Total: 50]